Why?
Expectations.
Expectations of them and their outcomes.
When I was in school (a sink comprehensive- ~10% got 5 A-Cs at GCSE) you went to school, you sat in a classroom and behaved (yeah- they'd have done the silence, hands up, anyone speaks you all troop into the corridor and practise coming in silently, putting your hands up as @NaturesEnd mentioned. We still had corporal punishment until Y9). You sat exams at the end of Y11, it was down to you if you prepared for them, you either passed or failed, then left or went to 6th form (at least 50% just left, and got jobs or got married and had children). There was no comeback, no pressure on the school or the teachers for anyone to pass, no performance tables in those days. No national curriculum even.
One girl in the whole school (1000+) had ED, none in my year. "Self harming" meant suicide attempt in those days, and those didn't happen either, the only case I know was a boy who'd had a life-changing diagnosis at 16.
It was in an area of severe poverty, but even though most ex-pupils didn't have qualifications, the vast majority went on to productive jobs or college that led to jobs later.
Since then, performance tables have been introduced, HTs and teachers are held responsible (by pupils, parents and govt) if children don't get 5A*-C GCSEs, no-one values vocational jobs or training, it's all about A levels and University, and everyone has to stay in education until 18, so there's enormous pressure to get onto the best 6th form courses, at all levels of ability. Then at 6th form, they have to work non-stop to get the best university places.
Pupils have to be constantly seen to be making progress, they have to work hard from the moment they begin Y7, there is no let up, they are assessed regularly, and it's followed up when things are on the slide, but the onus has shifted from the pupil to the teacher. It has become the teachers' responsibility to get children through their exams, and consequently schools have become places of intense pressure- if X % don't get their target grade (which is essentially determined by how good/crap their primary school was or how much the primary cheated on KS2 tests) teachers lose pay. If your school's results go down, parents slag you off all over SM, and pull their children out, you lose funding, have to cut staff, and amazingly can't improve results
Never mind the ability of that cohort of children...
Obviously, different areas have different issues in their schools, and many schools are doing the job of society/ social services in clothing and feeding children, providing and/or washing their uniforms, supporting parents apply for jobs/benefits/support, doing safe and well checks on children they haven't seen in school, etc.
Universally there are huge expectations upon schools to magically produce fully-functioning, fully qualified, all singing and dancing, well adjusted adults, on really quite stringent budgets.
But expectations of everything have changed since the 70s/80s in our society. Everyone expects to own a home, a car, to have foreign holidays, a wardrobe full of clothing for every occasion, satellite television, a high-end smartphone, a fridge full of copious amounts of tempting and delicious food, to go out as much as they like, to restaurants, the cinema, theatre, bars, coffee shops, etc etc etc.
No-one wants a hard life, with few comforts or pleasures, relentless struggle, no choices or opportunities. Why would they?